The present invention relates generally to valve devices for gases and more particularly to a valve device for gases of a type wherein, by turning in forward and reverse directions a main rotating shaft for controlling the degree of valve opening, a sliding member is caused to undergo displacement by rotations of the main rotating shaft thereby to vary controllably the valve opening degree.
Heretofore, as a valve device for gases of this kind wherein the degree of opening of a valve structure is controlled by rotating in forward and reverse directions a main rotating shaft for controlling the valve opening degree, there has been a device so adapted that the quantity of rotation of the main rotating shaft for valve opening degree control is detected to produce a detection signal, which is fed as input into a controller thereby to control the quantity of rotation of the main rotating shaft and thereby to control the valve opening degree to a desired setting value.
In this known device, the quantity of rotation of the main rotating shaft is controlled by direct detection thereof, and for this reason there have been problems such as the difficulty of accomplishing highly accurate and positive control of the valve opening degree.
In addition, in the prior art, there has been another device of a construction in which a limit switch is provided within the path of movement of a reciprocating control plate screw engaged through female threads with male threads provided on a main rotating shaft and is adapted to be closed when the reciprocating control plate reaches a predetermined position thereby to stop the rotation of the main rotating shaft and control the valve opening at a specific value.
In this known device, the reciprocating control plate is caused to move by direct screw driving action resulting from rotation of the main rotating shaft for valve opening control. For this reason, the range of movement of the reciprocating control plate is narrow, whereby there has been the difficulty of carrying out highly accurate and positive valve opening control.
Furthermore, in known devices of the instant kind, if the rotation of the main rotating shaft for valve opening control is continued even after it has reached its normal control limit point, excessive force will be exerted on the diaphragms of a pilot valve, and in extreme cases, the diaphragms may be ruptured.